This invention relates to papermaking or other industries where a web is produced, and more particularly to an apparatus for cooling the produced web or a machine component in contact with the web.
In the production of webs, such as paper, magnetic tape, laminates, etc., it is often desirable to control the temperature of either the web, or a machine component in contact with the web, for the purpose of controlling certain web properties which are directly or indirectly affected by the temperature of either the web or the machine components in contact with the web. In some cases it is sufficient to control the average temperature of the process component in a uniform cross-machine manner, while in other cases the temperature of the process component must be controlled independently at all points across its width, in suitable cross-machine increments, for the purpose of profiling a given web property which is affected by the temperature control.
At the "dry-end" of the web producing machine, after the dried web leaves the dryer-section, it is typically threaded through a calender-stack. A variation of the temperature profile of the rolls of the calender-stack can be utilized to alter the diameter of the rolls and in turn thereby control the web thickness or caliper profile of the sheet exiting the calender stack.
Typical systems in use today control the surface temperature of rolls of a calender stack by either controlled convective heating or cooling of the roll surface, or inductive heating of the outside radial layer of the roll. With convective heating or cooling, the applied heat-transfer fluid (typically air) is cooled or heated, respectively, after the fluid is applied to the roll surface. To provide for adequate heating or cooling of the roll surface, such systems typically consume 5 to 10 kilowatts of power per foot, at full output, with resulting efficiencies of 15 to 85 percent, depending upon the system design.
Following the application to the web of coating solutions, ink, laminating glues, or any other externally applied substances used for converting the raw web into a specialized product, it is often desirous to chill or cool the web for the purpose of "setting"or "curing" the applied substance. The web may also be cooled to provide a very thin layer of atmospheric condensation (as exists on a cool substance in a warm humid environment) on the web surface to insure that the coated or otherwise wet surface is mechanically "insulated" during contact with subsequent machine-component surfaces. This action is typically accomplished by the use of a "chilled" roll, which is internally cooled, and which is in physical contact with the web.
The constant contact of the chilled roll with a freshly coated or printed web (or other suitably converted web) may result in the build-up of the previously applied converting substance on the surface of the chilled roll. If such build-up is permitted to continue, the surface residue inevitably mars the passing web and diminishes the quality of the converted product. This surface residue can in some cases be kept to an acceptable level by applying a cleaning-blade or "doctor-blade" against the roll surface, across the full width of the roll. Such a cleaning blade scrapes the roll clean as it rotates. However, the resultant contact between the blade and the roll can lead to wearing of the roll surface, which in turn diminishes both the cleaning-performance of the blade and the uniformity of web cooling. In addition, the residue removed by the blade must be evacuated from the blade and its surroundings continuously, and this exercise proves to be difficult in practice.
At various points in the production of a converted or treated web it is necessary to apply precisely metered quantities of liquid suolutions to the surface of the web. In the application of such liquids it has been noted that the absorption properties of certain webs, i.e. the ability of the webs to absorb and retain the applied fluid, is also affected by the initial web temperature.
In addition to the temperature related aspects of web production described above, there are a plethora of other process variables which are affected by the web temperature. The drying rate of the web in the dryer section and the compressibility of the web entering the calender-stack (which affects the compression of the web in the calender stack in response to an applied load created by the contact of two calender rolls between which the web passes) are two such variables. The gloss imparted to the sheet through the calender stack is another example of a web temperature-dependent variable. Indeed, each of these variables is dependent upon other machine properties as well, but each variable can nevertheless be controlled to some degree by controlling the web temperature.
At the "wet-end" of the web production process, before the saturated web enters the dryer-section of the machine, the web passes through one or more mechanical presses formed by the contact of two heavily loaded rolls. The function of these mechanical presses is to remove as much water as possible from the web prior to the dryer-section, where the remainder of the web moisture is removed through evaporation. It is known in the art that any method which is capable of positionally altering the water-removal rate through the presses will afford a means to control the initial and hence final web moisture profile.
An accepted method of controlling press water-removal rates is that of web temperature variation. This method is based upon the principle that the water-drainage rate through the web, in the presses, is proportional to and increases with a decrease in web-water viscosity and surface tension, both of which decrease with increasing web temperature. The application of heat to the web by such common means as infra-red heating and steam application, can be used to selectively heat the web and increase the related web water-drainage rate through the presses, thereby affording a measure of web moisture profiling. As would be understood by one skilled in the art, the response, definition, and amplitude of adjustment of any closed-loop moisture control system employing such web heating methods would be improved by the addition of an apparatus capable of selectively cooling the web. In this way it would be possible to selectively heat or cool any position of the web, to the degree desired, thereby improving the performance of the moisture profiling action.
Moisture profiling techniques, in which moisture is applied to a web so that the moisture is absorbed by the web, are also used to profile a web. Known techniques, however, cannot provide the necessary fine degree of control, and thus are not suitable for many applications, especially moisture profiling of a "dry" sheet at the dry end of the machine.
It is therefore a principal object of the present invention to provide an apparatus and method for the non-contact cooling of a web or of machine components in contact with the web.
A further object of the present invention is to provide an apparatus and method for the non-contact cooling of a web or of the machine components in contact with the web through the use of an evaporative cooling technique which may be uniformly executed across the width of the machine or locally executed both with respect to the cross-machine position and magnitude of the cooling applied at that position.
Another object of the present invention is to provide an apparatus and method for cooling a web or machine components in contact with the web, in a simple efficient manner, which may be enacted uniformly across the machine-width, or sectionally executed in a profiling manner, as required by the specific application.
Still another object of the present invention is to provide an apparatus for the non-contact cooling of a web that may be located above or below the web at any point in the web production process, as required to control the web temperature profile at that point and thereby control a chosen web production variable that is influenced by the web temperature.
It is another object of the present invention to provide an apparatus which can be used to selectively cool and adjust the temperature of a saturated web, through evaporative cooling, in order to control the water-removal rate in the presses.
Yet another object of the present invention is to provide an apparatus which is capable of selectively cooling a web through evaporative-cooling, and which may be coupled with any suitable means capable of selectively heating the web, so as to provide for both selective heating and cooling of the web prior to the mechanical pressing of the web.
It is still another object of the present invention to provide an apparatus and method for selectively cooling by evaporative cooling, any desired portion of a web or mechanical component in contact with the web in the cross-machine direction, so as to allow for the cross-machine control of the temperature profile of the web or mechanical component in contact with the web.
An even further object of the present invention is to provide an apparatus and method for profiling a web at all stages of production, including the dry end of the machine, by adding moisture that is absorbed by the web with the profiling being controllable across the width of the web.